Abstract

Bladder cancer is the ninth most incident form of cancer in the United States and is an exposure-related disease which contributes significantly to morbidity and health care costs. Of concern is the fact that incidence rates have been stable in men in the last twenty years but have been increasing in women at a rate of 0.2% a year. Although tobacco use and occupational exposures have been linked to bladder cancer, much of the etiology of this disease remains unexplained. Epigenetic alterations including changes to cellular DNA methylation levels contribute to carcinogenesis and may serve as powerful biomarkers of this disease. This investigation sought to determine whether hypomethylation at the long interspersed nuclear elements (LINE1), reflective of the level of global DNA methylation, in peripheral blood-derived DNA is associated with increased risk of bladder cancer.

LINE1 methylation was measured from blood-derived DNA obtained from participants of a population-based incident case control study of bladder cancer in New Hampshire. Cases included white, 25-74 year olds, with a histopathologically confirmed diagnosis of bladder cancer while controls were selected from records of the Department of Transportation and the Medicare Program. Bisulfite-modified DNA was pyrosequenced to determine LINE1 methylation status; a total of 285 cases and 465 controls were evaluated for methylation and for a number of different risk factors for bladder cancer including arsenic.

Being in the lowest LINE1 methylation decile was associated with a 1.8-fold increased risk of bladder cancer (95% CI, 1.12-2.90) in models controlling for gender, age and smoking, with the association being stronger in women than in men (ORs = 2.48, 95% CI 1.19-5.17 in women and 1.47, 95% CI 0.79-2.74 in men). Amongst non-invasive cancers, being in the lowest LINE1 methylation decile conferred an almost two fold risk of bladder cancer (OR 1.94, 95% CI, 1.17-3.22) while the risk was non-significant amongst invasive cancers after adjusting for

gender, age and smoking. Amongst controls, women were more likely to have lower LINE1 methylation than men (p-value 0.04), and levels of arsenic in the 90th percentile were associated with reduced LINE1 methylation (p-value 0.04).

This investigation has identified a novel epigenetic marker of bladder cancer which has the potential for utility as a screening strategy for this disease, and which may aid in providing a more comprehensive understanding of the etiology of bladder cancer. In addition, this study shows an increased risk of LINE1 hypomethylation with high levels of arsenic, raising important questions about the nature of the causal pathway for alterations in DNA repeat methylation to contribute to cancer risk. These results also suggest that women demonstrate lower levels of LINE1 methylation, which may also aid in explaining the disparate incidence of this disease in men and women.